There are tons actually, but the thing is many don't always base theirs off fanart. Instead they make one of their own art and cosplay it at the convention so It's easy to miss sometimes. Namely for Pokemon who don't have a design that you could recognize it immediately by. As for putting it together, I say they base it off many things:Sewing Skill level, great eye for thrifting, amount of money they'd like to budget for a commissioner, and the design itself (what they'd like- ball-gown, top-hat, goggles, etc). You just need to figure out what goes well with you.
-Joker (Black Butler)